1. Field of the Invention
The present invention is related to radio frequency (RF) amplifiers. More particularly, the present invention is related to radio frequency power amplifiers used in wireless communication applications such as cellular base stations where signals with high peak to average ratios are generated and amplified.
2. Description of the Prior Art and Related Background Information
Most digitally modulated carrier signals used in modern telecommunication systems have an amplitude envelope showing a large peak to average ratio. In such systems, to preserve signal integrity and prevent transmitter spurious emissions, the amplifying device has to maintain linearity by having sufficient headroom for the signal peaks, albeit producing a modest average output power and therefore having a low efficiency. Hence, the amplifier efficiency and its linearity are practically mutually exclusive.
Even from the early days of AM broadcasting and in more recent complex transmission systems such as satellite communications, cable TV applications and cellular telephony, the carrier amplifiers have been mostly used in conjunction with some means of linearization to achieve the required performance. Feedback and in RF frequency bands, feedforward linearization are widely used linearization techniques. Analog predistortion has been used since the early days of satellite communication where frequency division multiple access (FDMA) systems were employed for sharing transponder bandwidth. In recent years, with the advent of digital signal processing (DSP), digital predistortion has received much attention.
Nonetheless, despite the significant efforts directed to linearization of RF power amplifiers, such techniques typically come at the expense of amplifier efficiency. Therefore, it is desirable to have additional techniques to achieve linearity and improve efficiency in RF amplifiers. Also, it is desirable to provide linearization techniques which may have reduced costs compared to the above known techniques, or which may improve performance, employed alone or in combination with the above techniques.